1. Field of the Invention
This invention relates generally to photovoltaic solar cells and, more specifically, to a high-efficiency multijunction photovoltaic solar cell device.
2. Description of the Prior Art
Photovoltaic cells, commonly known as solar cells, are essentially semiconductors that have the capability of converting electromagnetic energy, such as light or solar radiation, directly to electricity. Such semiconductors are usually characterized by solid crystalline structures that have energy band gaps between their valence electron bands and their conduction electron bands. Free electrons normally cannot exist or remain in these band gaps. However, when light is absorbed by the type of materials that characterize such photovoltaic cells, electrons that occupy low-energy states are excited to jump the band gap to unoccupied higher energy states. For example, when electrons in the valence band of a semiconductor absorb sufficient energy from photons of the solar radiation, they can jump the band gap to the higher energy conduction band.
Electrons so excited to higher energy states leave behind them unoccupied low-energy positions or holes. Such holes can shift from atom to atom in the crystal lattice; thus, the holes act as charge carriers, as do free electrons in the conduction band, and contribute to the crystal's conductivity. Therefore, most of the photons that are absorbed in the semiconductor give rise to such electron-hole pairs. These electron-hole pairs generate the photocurrent and, in turn, the photovoltage exhibited by solar cells.
The electron-hole pairs produced by the light would eventually recombine, and thereby convert to heat or a photon the energy initially used to jump the band gap, unless prohibited from doing so. Therefore, a local electric field is created in the semiconductor by doping or interfacing dissimilar materials to produce a space charge layer. This space charge layer separates the holes and electrons for use as charge carriers. Once separated, these collected hole and electron charge carriers produce a space charge that results in a voltage across the junction, which is the photovoltage. If these separated hole and charge carriers are allowed to flow through an external load, they constitute a photocurrent.
It is known that photon energies in excess of the threshold energy gap or band gap between the valence and conduction bands are usually dissipated as heat; thus they are wasted and do no useful work. More specifically, there is a fixed quantum of potential energy difference across the band gap in the semiconductor. For an electron in the lower energy valence band to be excited to jump the band gap to the higher energy conduction band, it has to absorb a sufficient quantum of energy, usually from an absorbed photon, with a value at least equal to the potential energy difference across the band gap.
The semiconductor is transparent to radiation with photon energy less than the band gap. On the other hand, if the electron absorbs more than the threshold quantum of energy, e.g., from a larger energy photon, it can jump the band gap. The excess of such absorbed energy over the threshold quantum required for the electron to jump the band gap results in an electron that is higher in energy than most of the other electrons in the conduction band. Electrons that have energy levels higher than the lower edge of the conduction band, i.e., the top edge of the band gap, are referred to as "hot electrons". For every electron excited out of its normal energy level, there is a corresponding "hole". Thus, for each hot electron there can be a corresponding hot hole; both are generally referred to as "hot carriers".
Hot carriers usually lose their excess energy to the host lattice very rapidly as heat. The process in which the hot carriers dissipate their excess energy to the host lattice and equilibrate with the lattice at ambient temperature is known as thermalization. As a result, such thermalization of hot carriers reduces the carriers in energy to the energy level at the edge of the conduction band. Since such thermalization normally occurs in about 10.sup.-12 seconds. In other words, the effective photovoltage of a single band gap semiconductor is limited by the band gap.
The practical effect of the limitation before this invention is that the semiconductor designer has to sacrifice efficiencies in one area to achieve them in another. Specifically, to capture as many photons from the spectrum of solar radiation as possible, the semiconductor has to be designed with a small band gap so that even photons from lower energy radiation can excite electrons to jump the band gap. However, in doing so, there are at least two negative effects that must be traded. First, the small band gap results in a low photovoltage device, thus low power output occurs. Second, the photons from higher energy radiation will produce many hot carriers with much excess energy that will be lost as heat upon almost immediate thermalization of these hot carriers to the edge of the conduction band. On the other hand, if the semiconductor is designed with a larger band gap to increase the photovoltage and reduce energy loss caused by thermalization of hot carriers, then the photons from lower energy radiation will not be absorbed. Consequently, in designing conventional single junction solar cells, it is necessary to balance these considerations and try to design a semiconductor with an optimum band gap, realizing that in the balance, there has to be a significant loss of energy from both large and small energy photons. Materials, such as silicon with a band gap of 1.1 eV and GaAs with a band gap of about 1.4 eV, are relatively inexpensive and are considered to be optimum solar energy conversion semiconductors for conventional single junction solar cells. However, a need still exists for a device that can capture and use a larger range of photon energies from the solar radiation spectrum, yet not sacrifice either photovoltage or excess energy loss to heat by thermalization of hot carriers.
Much work has been done in recent years to solve this problem by fabricating tandem or multijunction (cascade) solar cell structures in which a top cell has a larger band gap and absorbs the higher energy photons, while the lower energy photons pass through the top cell into a lower or bottom cell that has a smaller band gap to absorb lower energy radiation. The two cells are connected electrically in series. Some work has also been done on multijunction photovoltaic cells that have more than two cells connected in series; however, this invention is directed to two cell multijunction devices.
A number of characteristics are necessary to achieve efficient multijunction photovoltaic devices, an optimum combination of which have not been achieved before this invention. Such desirable characteristics include a monolithic multijunction device in which the upper and lower cells are current matched, i.e., absorb photons at the same rate, thus producing the same current. Further, the top cell and the junction between the top and bottom cells should be optically transparent to the lower energy radiation, while the electric current flow between the top and bottom cells should be substantially unimpeded. It is also very desirable, if not essential for economic considerations, that the cell materials be readily available, inexpensive, and relatively easy to fabricate.
It has been widely published and generally accepted by persons skilled in this art based on computer models and calculations that the most efficient, current matched multijunction tandem photovoltaic devices are achieved when the top cell has a band gap in the range of 1.50 to 1.75 eV and the bottom cell has a band gap in the range of 1.00 to 1.25 eV. See, for example, J. A. Hutchby, Robert J. Markunas, and Salah M. Bedair, "Material Aspects of the Fabrication of Multijunction Solar Cells", Center for Semiconductor Research, Research Triangle Park, NC, S.P.I.E. Journal (July 1985), and Fan, et al., "Optimal Design of High-Efficiency Tandem Cells", Proceedings of the 16th IEEE Photovoltaic Specialists Conference, pp. 692-701 (1982). The graph in FIG. 3 was published in both of the above-cited articles to illustrate this "target" band gap range for multijunction tandem photovoltaic devices.
It has also been generally accepted by persons skilled in the art that the desired optical transparency and current conductivity between the top and bottom cells in monolithic multijunction tandem devices would be best achieved by lattice matching the top cell material to the bottom cell material. Mismatches in the lattice constants create defects or dislocations in the crystal lattice where recombination centers can occur to cause the loss of photogenerated minority carriers, thus significantly degrading the photovoltaic quality of the device. More specifically, such effects will decrease the open-circuit voltage (V.sub.OC), short circuit current (J.sub.SC), and fill factor (FF), which represents the relationship or balance between current and voltage for effective power output.
In general, efforts and developments in the prior art have been directed toward multijunction tandem cells comprising semiconductor materials in the above-described "target" band gap ranges of 1.50 to 1.75 eV for the top cell and 1.00 to 1.25 eV for the bottom cell in the belief that such ranges offered the most promise for high efficiency devices. For example, the above-cited Hutchby, et. al., article on p. 4 suggests AlGaAs, GaAsP, and AlAsSb for top cells and Si, InGaAs, GaAsSb, InAsP, AlGaSb, and AlInSb for bottom cells. For quaternary alloys, the Hutchby, et. al., article suggests AlGaInP, AlGaInAs, and AlGaAsSb for top cells and InGaAsP, AlGaAsSb, and AlGaInSb for bottom cells. Although, these alloys have band gaps in the target ranges, they are not very well lattice matched, either among these materials themselves or with any inexpensive, readily available substrate material.
Likewise, the above-cited Fan, et al., article extensively discusses the potential use of compositions with band gaps in these "target" band gap ranges. The Fan et. al., article like the Hutchby et. al, article discussed above, directed primary efforts toward minimizing effects of lattice mismatches that cannot be avoided in those materials having band gaps in the "target" ranges.
U.S. Pat. No. 4,255,211 and U.S. Pat. No. 4,332,974, both issued to L. Fraas, disclose the use of a germanium substrate with a G.sub.0.88 In.sub.0.12 As (1.25 eV) bottom cell and Ga.sub.0.43 In.sub.0.57 P (1.75 eV) top cell deposited thereon. Again, these prior art tandem devices designed by Fraas are directed to the previously-described "target" range band gaps of 1.00 to 1.25 eV for the bottom cell and 1.50 to 1.75 eV for the top cell, while lattice matching is apparently considered secondary. For example, both of the above-cited U.S. Pat. Nos. 4,255,211 and 4,332,974 specify that these materials are only lattice matched to a range of .+-.1%, even in the three-layered, quaternary alloy Ga.sub.0.88 In.sub.0.12 As/Ga.sub.0.69 In.sub.0.31 As.sub.0.5 P.sub.0.5 /In.sub.0.5 Ga.sub.0.5 P structure described in U.S. Pat. No. 4,332,974. However, it has now been determined that even lattice mismatching as low as .+-.0.01% causes significant degradation of photovoltaic quality, which provides some indication why such prior art devices directed to the above-described target band gap ranges do not perform as efficiently as initially expected by persons skilled in the art in spite of their above-described efforts to minimize those effects.
The previously-cited Hutchby, et al, article also discusses the near-perfect lattice-matched cell/substrate structures of AlGaAs/GaAs and GaAs/Ge, which avoid the problems of non-lattice-matched structures. However, GaAs and Ge are not current-matched, so they also do not provide the efficiencies desired. By nature of the series connection, the output current of the device is limited to the smaller of the current producing capabilities of the two cells. The GaAs/AlGaAs structures, on the other hand, are both lattice-matched and current matched; however, efficiency results are inconsistent because of problems with AlGaAs near the direct-to-indirect composition at the heterojunction interface.
Because of the lattice mismatching and/or current mismatching of the prior art multijunction photovoltaic devices, as described above, efficiencies have not approached the theoretical maximum or target of 30 to 36% at AM1. Therefore, a need remains for a current-matched and lattice-matched multijunction solar cell device that does not have the problems described earlier.